1. Field of the Invention
The present invention relates to an electrophotographic image forming apparatus in which an electrostatic latent image, which is formed on an electrophotographic photosensitive member by bringing a developer carrying member into contact with the electrophotographic photosensitive member, is developed with developer.
The electrophotographic image forming apparatus is herein configured to form an image onto a recording medium (such as recording sheet, OHP sheet, and cloth) with an electrophotographic image forming process. Examples of the electrophotographic image forming apparatus include an electrophotographic copying machine, an electrophotographic printer (such as laser beam printer and LED printer), a facsimile machine, and a multifunction peripheral (multifunction printer) having functions of those apparatus.
2. Description of the Related Art
In recent years, a color image forming apparatus for forming a color image with developers of a plurality of colors has become widely spread. As the color image forming apparatus, there has been known what is called an in-line type image forming apparatus including photosensitive drums which correspond respectively to image forming operations respectively using the developers of a plurality of colors and are disposed in line along a moving direction of a surface of a transfer-object member, onto which toner images are to be transferred. The in-line type image formation is preferred in ease of meeting demands for an increase in speed of image formation, extensive application to the multifunction printer.
In some of the in-line type image forming apparatus, the plurality of photosensitive members are disposed below an intermediate transfer member as a member on which an image is to be transferred or a recording material carrying member for conveying a recording material as a member on which an image is to be transferred. When the plurality of photosensitive members are disposed below the intermediate transfer member or the recording material carrying member, for example, a fixing device and a developing device (or exposure device) can be disposed at positions separate from each other while interposing the intermediate transfer member or the recording material carrying member therebetween in a main body of the image forming apparatus. Thus, the developing device (or exposure device) is hard to be influenced by heat of the fixing device, which is an advantage of the in-line type image forming apparatus.
In many of the color image forming apparatus, a contact developing process is widely employed in terms of advantages of high reproducibility of a halftone image and marked suppression of an excessive edge effect of an image. The contact developing process refers to a process in which the developer carrying member of a developing unit is brought into contact with the electrophotographic photosensitive member and a latent image formed on the electrophotographic photosensitive member is developed with developer (toner).
In a general configuration for the contact developing process, one of the electrophotographic photosensitive member and the developer carrying member is formed of an elastic member (including sheet backed up by the elastic member) and another of the electrophotographic photosensitive member and the developer carrying member is formed of a rigid member. With this, the electrophotographic photosensitive member and the developer carrying member are firmly fitted to each other uniformly in directions of rotation axes of the electrophotographic photosensitive member and the developer carrying member. In the simplest configuration, a photosensitive drum obtained by coating an outer peripheral surface of an aluminum cylinder with a photosensitive layer is used as the electrophotographic photosensitive member, and a developing roller formed of a rubber elastic member is used as the developer carrying member. In particular, when a process cartridge removably mounted to the main body of the image forming apparatus includes the electrophotographic photosensitive member and the developer carrying member, in many cases, the electrophotographic photosensitive member and the developer carrying member are used in the above-mentioned combination.
Further, the developing roller is brought into contact with a toner feed roller (elastic roller) as a developer feed member configured to feed the developer (toner). With this, toner is fed to the developing roller, and non-developed residual toner is scraped off.
When development is performed by using the contact developing process, the photosensitive drum and the developing roller are driven and rotated while being in contact with each other. Thus, when the contact state is continued unnecessarily long, both the photosensitive drum and the developing roller are abraded against each other due to friction. As a result, the photosensitive drum and the developing roller earlier reach their end of life.
As a countermeasure, as disclosed in Japanese Patent Application Laid-Open No. 2006-292868, there has been proposed a configuration in which the photosensitive drum and the developing roller are allowed to be brought into contact with and separated from each other so that the developing roller is separated from the photosensitive drum during non-development.
However, there may occur a phenomenon called fog, in which toner is transferred from the developing roller onto a non-image portion on the photosensitive drum when the developing roller and the photosensitive drum separated from each other are brought into contact with each other. The fog is caused by insufficient triboelectrification of the toner.
Meanwhile, through repetitive revolutions of the developing roller, the toner carried by the developing roller becomes charged triboelectrically. Thus, an electric charge amount of the toner increases to an appropriate value, with the result that the fog phenomenon described above is suppressed. However, in this case, the developing roller is rotated over a longer period of time, and hence the problem that the developing roller earlier reaches its end of life occurs as described above.
In particular, as for a counter rotation configuration in which the developing roller and the toner feed roller are rotated in directions reverse to each other at a contact portion therebetween, the toner feed roller is rotated slower than the developing roller. Thus, in order to refresh toner captured in the toner feed roller by causing the toner feed roller to rotate one revolution, the developing roller needs to rotate one or more revolutions. As a result, the number of revolutions of the developing roller increases, which causes a problem that the developing roller much earlier reaches its end of life.